1. Field of the Invention
This invention relates to a negative dot image-forming method using a silver halide photographic light-sensitive material and more particularly to a stable activator type developing method which permits a dot image of good dot quality and screen range to be formed by use of a hydrazine compound.
2. Description of the Prior Art
In printing an original of continuous gradation by use of an off-set plate or the like, the tone is reproduced by a collection of big and small points called "dots". These dots are very minute and are present in a number of 80 to 200 per square inch, and moreover they are required to be sharp individually. In the printing industry, therefore, a combination of a lith type light-sensitive material and a lith developer is employed, which enables to form a dot image of greatly high contrast by a specific development effect called a "lith effect".
The lith developer is an alkaline solution wherein the concentration of a sulfite acting as a preservative is generally controlled to extremely low levels and only hydroquinone is used as a developing agent. Where the lith type light-sensitive material is developed with this solution, the tone of the lith type light-sensitive material will generally be higher in contrast with a decrease in the concentration of sulfite ions.
However, since the general properties of the lith type light-sensitive material are greatly influenced by the concentration of the developing agent and are sensitive to changes in the concentration of bromine ions, it is difficult to steadily obtain an image of constant quality. Moreover, because of the markedly low concentration of the sulfite ions as preservatives in the lith developer, the lith developer after being prepared is very low in its resistance to oxygen in air and it is disadvantageously vigorously deteriorated.
Furthermore, in continuously processing the lith type light-sensitive material, the bromine ion is released from an emulsion layer and the developing agent is consumed as is the case with typical silver halide light-sensitive materials. Therefore, even if they are supplemented, it is necessary to check and correct the activity of the developer every several hours. This leads to troublesome or complicated daily production control.
In addition, in processing by such conventional methods, a long development time of from one minute to two minutes at a development temperature of from 25.degree. C. to 35.degree. C. has been needed to obtain sufficient blackening density and dot quality.
Therefore, those methods have eagerly been desired which are able to provide dot images of super high contrast and of good dot quality and screen range.
Japanese patent application (OPI) No. 22438/1976 discloses a method in which in order to avoid the use of the unstable lith developer, a hydroquinone based developing agent is introduced in a silver halide emulsion and the processing is carried out by use of an alkaline activator in the presence of a hydrazine compound such as hydrazine sulfate to obtain a negative image of high contrast.
This method improves the stability of the processing solution and accelerates the processing rate. This method, however, has the disadvantages that the dot quality obtained is inferior to those of conventional lith type light-sensitive materials, the dot characteristics suitable for use in the plate-making using a contact screen cannot be obtained, and that the screen range is of too high contrast, although the contrast characteristics close to those of the lith type light-sensitive material can be obtained. Moreover, for light-sensitive materials in which hydrazine compounds containing a NH.sub.2 NH-- group have been introduced, it is difficult to hold the contrast characteristics obtained at the beginning of the production of the light-sensitive materials for a long period of time which is commercially required. This seems due to the vigorous decomposition of the hydrazine compounds with time. Therefore, no light-sensitive materials capable of providing images of high contrast can be obtained by such a method in which the hydrazine compounds of the type as described above are incorporated in the light-sensitive materials.
U.S. Pat. No. 2,419,975 discloses a method in which a hydrazine compound is added to a silver halide emulsion to obtain a negative image of high contrast. It is described that when the hydrazine compound is added to the silver chlorobromide emulsion and the development is carried out using a developer having a pH value as high as 12.8, the photographic characteristics of markedly high contrast with a .gamma. larger than 10 can be obtained. However, many of the hydrazine compounds as disclosed in this patent are of low stability in the light-sensitive materials and cannot be stored for extended periods of time. For strongly alkaline developers having pH values close to 13, developing agents are easily oxidized by air and unstable, and they cannot be stored or used for extended periods of time. Moreover, the development time is nearly equal to those of conventional lith development. Furthermore, for use in the application of the plate-making using a contact screen, such images having only the photographic characteristics of high contrast wherein .gamma. is 10 or more are inferior in dot quality, are of too high contrast in screen range, and are not sufficiently satisfactory.
The inventors have developed a method to obtain the photographic characteristics preferred for the reproduction of dot images in which a stable developer is used, and disclosed in Japanese patent application (OPI) No. 37732/1979 (corresponding to U.S. patent application Ser. No. 934,785 filed on Aug. 18, 1978) now U.S. Pat. No. 4,221,857.
According to this method, there can be obtained the good quality which is higher in density than the dot quality obtained by a conventional combination of the lith type photographic light-sensitive material and the lith developer and which is free from fringe. However, no screen range of low contrast could be obtained as in the case where the conventional lith type development is applied.
Hereinafter, the dot quality and the screen range will be explained in detail.
The term "dot quality" means the performance of points when the blackening density is converted through a contact screen in the corresponding point area and, in general, those of lesser fringe are preferred.
The screen range indicates the changes of the dot area relative to the exposure amount. Theoretically it is of the character to be determined by the density pattern of the contact screen used.
Therefore, even by the methods as described in the above cited references, if a contact screen having a density pattern suitable for a light-sensitive material to be used is chosen and used, the desirable gradation will be obtained. However, such choice of the suitable contact screen according to the type of the light-sensitive material used is undesirably very troublesome for those practically engaged in the operation of the plate-making.
Thus it has long been desired to produce light-sensitive materials which permit the formation of good dots, which are of lesser fringe, by use of a stable processing solution and furthermore which enable one to produce the practically same screen range by use of the same contact screen as used in the conventional lith development without employing such a special operation as the choice of the contact screen.